NIH
There was a significant inhibition of tumor burden, in both the tumor model system studied (from P < 0.005 to P < 0.001). Tumor incidence was also reduced by both the doses of Azadirachta indica extract.

The prospects of relatively cheaper cancer drugs could then be brighter, particularly for the under-privileged cancer patients of the world.

Note: beware that cheap versions of this food supplement. It sometimes comes with heavy metals. So search for a good brand and that can certify it has been tested. Also it is not for pregnant women.
see NIH review of neem (toxicity form the seed is described here) Leave is relatively safe to consume in small amounts but consult your physician if you decide to add this to your diet.http://www.ncbi.nlm.nih.gov/pubmed/15261960